1. Field of the Invention
This invention relates generally to a vacuum circuit interrupter, and more particularly to a structure of a vacuum circuit interrupter of low surge and large current interruption capacity.
2. Description of the Prior Art
In general, high voltage resistance, high fusion-bonding resistance, low surge and large current interrupting capacity are demanded of a vacuum circuit interrupter. These characteristics are highly dependent on the type of materials used for the contact plates of the circuit interrupter. There are no ideal materials for contact plates with which all these characteristics are achieved. Therefore, for most circuit interrupters in which the interrupted current exceeds 4 kA (root mean square), sophisticated means such as addition of spiral electrodes and vertical magnetic field electrodes are employed, as disclosed in U.S. Pat. No. 4,367,382. In such a circuit interrupter, magnetic force generated by the current drives the arc to move around, which prevents local heating of the electrodes.
In recent years, power supply systems have become more complex, and there is an increasing number of situations involving opening and closing of large inductive loads. Therefore, the need for resolving problems of surging is increasing. Harmful surges may be suppressed by adding a surge absorber outside a vacuum circuit interrupter. However, if an outside surge absorber is utilized, the total system of the vacuum circuit interrupter requires a large space for installation, which makes the circuit interrupter bulky and less desirable.
Japanese Patent Disclosure (Kokai) No. 60-243919 shows a low surge vacuum interrupter which includes an electrode having an Ag-WC low surge contact plate. A copper arc plate has a diameter larger than that of the contact plate, and the surface of the copper arc plate is silver plated. A coil is provided at the rear surface of the arc plate. The electrode constitutes a so-called vertical magnetic field electrode structure, in which a magnetic field is imposed parallel to the arc at the time of interruption.
As is well-known, it is important that the arc be dispersed uniformly over the whole surface of the electrodes for interruption of a large current, larger than approximately 10 kA. However, the stabilized arc voltage is about 20 V for contact plates of Ag-WC, while the arc voltage for copper arc plates is about 30 V. Therefore, it is difficult for an arc to move from the contact plate to the copper arc plate. In order to deal with this problem, silver plating was effected over the copper arc plate to facilitate movement of arcs from the contact plate to the copper arc plate, which brought about uniform dispersion of the arc over the whole front surface of the electrode and provided a circuit interrupter with a good interrupting performance.
When a small current, smaller than approximately 10 kA, is interrupted, surge may take place. In this case, however, the arc stays on the low surge contact plate, and local heating on the electrode does not create any problem.
In recent years, there has come to be a demand for improvements of circuit interrupters to provide large current interrupting repetition capacity especially in the regions where short circuiting incidents are frequent. With the conventional electrode described above, the arc which appears at the time of interruption of large currents travels from the contact plate to the silver plated arc plate and causes scattering and vaporization of the silver plating. Therefore, as the number of times of interruptions increases, the silver on the surface of the copper arc plate is scattered away. It is technically and economically difficult to have thick silver plating on the copper arc plate which endures for a sufficiently long time. When the silver plating has removed from the arc plate, the arc voltage of the arc plate rises to a copper arc voltage level, which makes it difficult for the arc to travel from the contact plate to the copper arc plate. In such a situation the arc is no longer uniformly dispersed.